Atrium detection, battery autonomy and interfaces on an L2 fire alarm system

What L2 actually requires

L2 provides detection in escape routes and in rooms or areas adjoining escape routes, plus areas of special fire risk. It is not full coverage of every space. The question is therefore whether the atrium is part of the escape route system - and here it plainly is, because the main escape stair discharges into it. That makes it an escape route for L2 purposes, and detection is required.

Why the contractor's reason fails

Maintenance access is a design constraint, not a justification for omitting detection. The correct response is to select a technology suited to the height. Point smoke detectors are limited to roughly 10.5 m mounting height by the standard; an 18 m atrium is well beyond that, and a point detector at that height would also be unreliable in terms of smoke reaching it. Beam smoke detectors, or aspirating detection where appropriate, are designed for these heights and are the conventional solution, provided the manufacturer's data supports the height and the beam paths are set out across the space - including at the top of the atrium, at intermediate levels and at the base where the stair discharges.

Design issues to resolve

• Smoke stratification in a tall, possibly warm atrium can prevent smoke reaching the top; the detector layout must consider this and may need intermediate levels.
• Any detection in the atrium must be supplemented by the cause-and-effect: the atrium is likely a large volume within the same smoke control zone as the stair, so the response of smoke ventilation and the door release must be defined.
• If an alternative approach is proposed, it must be as a documented performance-based solution accepted by the fire engineer and the approving authority, not as a cost-saving omission at a design review.

Recommend refusing the omission and requiring a beam or aspirating detection design for the atrium with manufacturer's evidence and a maintenance access strategy.

Calculation

Required capacity, before ageing:

C = 1.25 × (I_quiescent × 24 h + I_alarm × 0.5 h) C = 1.25 × (1.2 × 24 + 3.5 × 0.5) C = 1.25 × (28.8 + 1.75) C = 1.25 × 30.55 = 38.2 Ah

So the 38 Ah battery is already marginal before any allowance for ageing. Apply the end-of-life factor (typically the battery is replaced at 80% of rated capacity): 38 × 0.8 = 30.4 Ah available, against a requirement of 38.2 Ah. The battery is undersized and will fail the 24 h standby requirement well before its nominal life is over.

What to require

1. A battery calculation sheet using the measured quiescent current of the installed system (panel, detectors, sounders, interfaces, door holders), not a generic figure, and the alarm current with every sounder operating.
2. Sizing at the end-of-life capacity, and at the expected ambient temperature - battery capacity falls with temperature and rises with it in terms of ageing, so the enclosure environment matters.
3. Selection of the next standard size above the calculated value - likely 2 × 12 V 45 Ah or 65 Ah depending on the final figures - and confirmation that the enclosure and charger are rated for it.
4. Confirmation that the charger can recharge the battery to 80% within the standard recharge period while the system is in quiescent operation.
5. A sounder circuit voltage-drop check at end-of-discharge battery voltage, not at nominal 24 V, since that is where sounder circuits fail to meet their output.

The point to make to the contractor is that the 1.25 factor and the ageing allowance are not optional extras; the calculation as presented stops one step short of the answer.

AHU shutdown

Driving AHUs from a volt-free contact from the panel is acceptable in principle, but the trigger must be defined by the cause-and-effect matrix and by the smoke control strategy - shutting down an AHU that is part of a smoke clearance system is a serious error. Require: a documented cause-and-effect for each AHU, monitored signal paths where the standard requires them, and a clear interface drawing showing whether the contact is from the fire alarm panel or via a dedicated relay module, with failure of the signal itself annunciated as a fault.

Door hold-opens

Feeding them from the nearest lighting circuit is not acceptable. BS 7273-4 requires the release to be fail-safe and the circuit to be monitored; a lighting circuit that is switched off or isolated will release the doors without any indication, and the isolation cannot be controlled. Require the magnets to be supplied from a monitored circuit dedicated to the door release function, with local isolation clearly labelled and monitored, or from the fire alarm system's dedicated 24 V supply where the manufacturer's design permits. Each door should also be checked for the correct release on both fire alarm and power failure.

Sprinkler flow switch

The flow switch must be connected so that both alarm and fault conditions are signalled. A short or open circuit on the wiring must produce a fault at the panel, not silence. This normally means an end-of-line monitored input or an addressable module, and in many jurisdictions a valve-monitor switch as well, so that a closed valve is also annunciated. Connecting it as an alarm input only means the system can lose its sprinkler signal entirely without anyone knowing - probably the single most consequential defect in the list.

For all three, require revised interface drawings, updated cause-and-effect, and a witnessed interface test at commissioning before acceptance.